Baseboard trim is usually installed last, so that it can be joined to the casings and the wainscoting. It is preferable to install it after a wood finish floor is laid. If this is not possible, the baseboard will have to be elevated with scraps of the flooring to give the necessary clearance. Baseboard is invariably installed before carpeting is laid.
It’s a good idea to pre-finish baseboard before you install it. Use stain or a clear finish on hardwood and at least a prime coat of paint on softwood. Filling and touching up or final painting can be done later.
The joinery depends on the type of baseboard you use. Flat baseboard with no decorative profile can simply be butted at the corners. Molded base will have to be coped or mitered at inside corners and mitered at outside corners. A coped inside corner is superior to a miter, because it is less likely to open up after it is nailed.
Baseboard trim can be made up of from one to three separate pieces. Ideally, the baseboard should not be thicker than any door casing to which it butts up. If it is thicker, butt it up to the point where it meets the casing and then cut remaining thickness back at a 45-degree angle. If you are installing a base shoe along the bottom edge of the baseboard, nail the baseboard to the wall ¼ inch from the top edge. Nail it again ½ inch above the finish floor, position the base shoe over the joint (allowing a 1/8-inch expansion gap between the base shoe and the floor), and attach the base shoe only to the baseboard. This allows the floor to expand and contract without pushing and pulling the trim.
As with any horizontal, or running, trim, prepare a plan view sketch of the room and determine the installation sequence. Arrange the cuts so that the joints are out of the direct line of sight as you enter the room. Install the first piece of trim on the longest wall opposite the door, using square cuts at each end. Move progressively around the room as you did with ceiling trim.
When working with outside corners, it is helpful to use scraps of baseboard to mark the intersection lines on the floor, to determine the angle of cut on the miter. After coping or mitering one end of the baseboard and fitting it snugly to the inside corner, mark the baseboard on top at the outside corner of the wall and on the bottom at the intersection lines on the floor. With a combination square, draw a vertical line up the face of the baseboard to the top edge. Connect this line across the top at an angle to the mark at the back edge of the piece. Cut to this line and set the piece aside. Prepare the mating piece using the same method, but make the cut about 1/16 inch longer than the intersection mark on the bottom of the piece. Install and nail the first piece; then fit this second piece, making adjustment cuts as necessary. Install it and nail it in place.